Supplement to Nature ^i 



No. 2801 



JULY 7, 1923 



The Structure 



By Prof. 



The General Picture of the Atom. 



THE present state of atomic theory is characterised 

 by the fact that we not only beh'eve the existence 

 of atoms to be proved beyond a doubt, but also we 

 even believe that we have an intimate knowledge of the 

 constituents of the individual atoms. I cannot on 

 this occasion give a survey of the scientific develop- 

 ments that have led to this result ; I will only recall 

 the discovery of the electron towards the close of the 

 last century, which furnished the direct verification 

 and led to a conclusive formulation of the conception 

 of the atomic nature of electricity which had evolved 

 since the discovery by Faraday of the fundamental 

 laws of electrolysis and Berzelius's electrochemical 

 theory, and had its greatest triumph in the electrolytic 

 dissociation theory of Arrhenius. This discovery of 

 the electron and elucidation of its properties was the 

 result of the work of a large number of investigators, 

 among whom Lenard and J. J. Thomson may be 

 particularly mentioned. The latter especially has 

 made very important contributions to our subject by 

 his ingenious attempts to develop ideas about atomic 

 constitution on the basis of the electron theory. The 

 present state of our knowledge of the elements of 

 atomic structure was reached, however, by the dis- 

 covery of the atomic nucleus, which we owe to Ruther- 

 ford, whose work on the radioactive substances 

 discovered towards the close of the last century has 

 much enriched physical and chemical science. 



According to our present conceptions, an atom of 

 an element is built up of a nucleus that has a positive 

 electrical charge and is the seat of by far the greatest 

 part of the atomic mass, together with a number of 

 electrons, all having the same negative charge and 

 mass, which move at distances from the nucleus that 

 are very great compared to the dimensions of the 

 nucleus or of the electrons themselves. In this picture 

 we at once see a striking resemblance to a planetary 

 system, such as we have in our own solar system. 

 Just as the simplicity of the laws that govern the 

 motions of the solar system is intimately connected 

 with the circumstance that the dimensions of the 



' Lecture delivered at Stockholm, December 11, 1922, on the occasion 

 of the receipt of the Nobel prize in physics for the year 1922. English 

 translation by Dr. Frank C. Hoyt. 



of the Atom.i 



N. Bohr. 



moving bodies are small in relation to the orbits, so 

 the corresponding relations in atomic structure provide 

 us with an explanation of an essential feature of 

 natural phenomena in so far as these depend on the 

 properties of the elements. It makes clear at once 

 that these properties can be divided into two sharply 

 distinguished classes. 



To the first class belong most of the ordinary 

 physical and chemical properties of substances, such as 

 their state of aggregation, colour, and chemical re- 

 activity. These properties depend on the motion of 

 the electron system and the way in which this motion 

 changes under the influence of different external 

 actions. On account of the large mass of the nucleus 

 relative to that of the electrons and its smallness in 

 comparison to the electron orbits, the electronic 

 motion will depend only to a very small extent on the 

 nuclear mass, and will be determined to a close 

 approximation solely by the total electrical charge 

 of the nucleus. Especially the inner structure of the 

 nucleus and the way in which the charges and masses 

 are distributed among its separate particles will have 

 a vanishingly small influence on the motion of the 

 electron system surrounding the nucleus. On the 

 other hand, the structure of the nucleus will be 

 responsible for the second class of properties that 

 are shown in the radioactivity of substances. In the 

 radioactive processes we meet with an explosion of 

 the nucleus, whereby positive or negative particles, 

 the so-called a- and /3-particles, are expelled with very 

 great velocities. 



Our conceptions of atomic structure afford us/ 

 therefore, an immediate explanation of the complete 

 lack of interdependence between the two classes of 

 properties, which is most strikingly shown in the 

 existence of substances which have to an extraordinarily 

 close approximation the same ordinary physical and 

 chemical properties, even though the atomic weights 

 are not the same, and the radioactive properties are 

 completely different. Such substances, of the existence 

 of which the first evidence was found in the work of 

 Soddy and other investigators on the chemical properties 

 of the radioactive elements, are called isotopes, with 

 reference to the classification of the elements according 

 to ordinary physical and chemical properties. It is 



